Thousands of satellites have been launched into earth orbit with nearly 1,000 of them still active and providing valuable services such as military and intelligence data collection, global positioning, telecommunications, weather and climate monitoring, and so on. These active satellites, however, are increasingly at risk of colliding with “space junk.” Space junk (“junk objects”) is the collection of now-useless, human-created objects in earth orbit such as spent rocket stages, defunct satellites, lost astronaut tools, and fragments from collisions. The risk of a collision occurring is more than a theoretical possibility as in February 2009 a collision occurred between an American Iridium satellite and a defunct Russian Cosmos satellite.
The risk of future collisions occurring is increasing because the density of objects in earth orbit is increasing. Although space junk will eventually be removed from orbit by the frictional forces of the earth's atmosphere, space junk can remain in orbit a very long time as those frictional forces are small. Space objects (e.g., active satellites and space junk) of certain types tend to orbit in certain orbital regions. For example, low-earth orbit (i.e., 160 km to 2,000 km) tends to contain space stations, upper rocket stages, and amateur satellites; middle-earth orbit (2,000 km to 35,876 km) contains navigation satellites; and the orbit at 36,000 km contains geostationary satellites. A critical density occurs when space objects enter orbit faster than space objects leave orbit. Eventually, some orbital regions may become so crowded that placing new satellites in orbit will be impractical as the risk of a collision will be too high.
Estimates place the number of junk objects at tens of millions. The vast majority of the junk objects are very small particles such as dust from solid rocket motors or paint that flakes off of satellites. A collision between an active satellite and such small junk objects can have an erosive effect, similar to sandblasting, on the satellite. Estimates place the number of junk objects in low-earth orbit that are larger than 1 cm to be around 300,000. A collision between an active satellite and such junk objects can have a more serious effect, but not necessarily a catastrophic effect. A significant number of junk objects, however, are larger than 10 cm, and a collision between an active satellite and such large space junk can indeed be catastrophic. The only way to avoid an impending collision with large space junk is to maneuver the satellite away from the space junk.
If a collision between an active satellite and large space junk were to occur, the result might be hundreds of fragments, some of which could be larger than a softball. The collision between the Iridium satellite and the Cosmos satellite produced thousands of junk objects that are still in orbit. Actions taken by certain countries have resulted in significantly increasing the amount of space junk. In 2007, China performed an anti-satellite weapons test that destroyed an aging weather satellite using a kill vehicle launched on board a ballistic missile. The destruction of the weather satellite created 2,000 baseball-sized or larger junk objects that could destroy a satellite and over 2 million junk objects that could cause damage to a satellite.
To help predict collisions so that evasive actions can be taken, the U.S. Joint Space Operations Center (“JSpOC”) gathers ground-based observations of space junk. The primary source of these observations is the Space Surveillance Network, which is a global network of sensors (29 optical telescopes and radars) operated by the U.S. Air Force. The Space Surveillance Network follows some 20,000 space objects the size of a baseball or larger, which can destroy a satellite.
Every day the JSpOC makes observations of space objects via its sensors, generates orbital models for the observed space objects, and makes collision predictions based on those orbital models. The observations for a space object that are collected over time are used to generate the orbital models. The day before making the observations, the list of space objects to be observed is prioritized giving manned satellites (e.g., the International Space Station) priority, followed by military and intelligence satellites, and so on. The JSpOC determines how many tracks are needed to determine the orbit of each space object in the list based primarily on the type and size of the space object and the change rate of its orbit. The sensors are then programmed to make the required observations of the space objects.
After the observations for a day are collected, the JSpOC collects the observations, generates orbital models for the space objects, and predicts what space object might collide in typically a 72- to 96-hour window. Satellite operators can use these predictions to maneuver their satellites to avoid the collision.
Unfortunately, the accuracy of the orbital models is insufficient to make collision predictions with an acceptable degree of certainty. A typical collision prediction may be based on the space objects passing within 1 km of each other. If satellite operators maneuver their satellites based on every such prediction, the satellites would quickly use up the available fuel needed to maintain their orbits. At an accuracy level of 1 km, the Iridium satellite constellation of 90 satellites providing voice and data to phones, pagers, and transceivers would be warned to move 10 satellites per day on average. As a result these warnings are typically ignored, with sometimes catastrophic results.
It would be desirable to have a system that would provide more accurate collision predictions for space objects and provide them in enough time so that space operators could take action to maneuver an active satellite to avoid the collision.